Modern electronic devices have high requirements with respect to transient response from a power supply. For example, microprocessors typically require that the power should respond nearly instantly to load variation. Multiple prior art methods have been developed in the controller of the DC-DC converter to achieve stable powering during transient load variation.
Pulse width modulation (PWM) is a basic control approach in DC-DC converters. FIG. 1 shows a trailing-edge PWM control waveform for peak current mode control. The peak current mode control has advantages over voltage mode control, including good linear modulation, simple network compensation and so on. As seen in FIG. 1, the upper waveform is a compensated saw-tooth signal Vsaw+Vc which is the sum of the compensation signal and the saw-tooth signal. At the leading edge of the compensated saw-tooth signal, the PWM signal is set HIGH. When the current signal reaches the compensated saw-tooth signal, the PWM signal is set LOW wherein the current signal is a feedback of the output current. In this control method, the PWM is set HIGH only at the leading edge and cannot respond quickly to the load variation. In another words, between the time of when PWM is set LOW and the time of the leading edge appearing, if load variation occurs, the PWM signal cannot respond until the next leading edge.
If a load transient variation occurs at the time that PWM is set low, the response delay is (1−D)*T, where D is the duty cycle, and T is the time of a cycle. When D is small, the delay can be close to one cycle. Thus, a new PWM control approach is required which has the benefits of current mode control, and also quick transient response.